A gun control group founded by former U.S. Rep. Gabby Giffords asked two web hosting companies on Friday to shut down websites selling parts and machines that help make untraceable homemade firearms known as “ghost guns.” The Giffords Law Center to Prevent Gun Violence asked the providers that host GhostGunner.net and GhostGuns.com to disable the websites for violating the hosting companies’ terms of service. The sites sell kits, components and machines that help create homemade semi-automatic weapons. It’s legal to build a gun in a home or a workshop, and advances in 3-D printing and milling have made it easier...